Smoking cessation intervention websites are now reaching millions of adults 24 hours a day. Unfortunately, currently available smoking cessation websites have low effectiveness. Their average quit rates are 7 to 10%. Now is the time to test a new intervention model with potential to boost quit rates of smoking cessation websites. To start a new research paradigm focusing on innovative web-based intervention content, the current study will test a promising treatment approach called Acceptance & Commitment Therapy (ACT). We recently developed the first ACT smoking cessation website and pilot tested it in a randomized controlled trial comparing it with a national-level current standard website following US Clinical Practice Guidelines (Smokefree.gov). The trial design was proven feasible, with successful national recruitment (N = 222) and balanced randomization. The ACT intervention was feasible, with higher participant utilization and satisfaction, as compared to Smokefree.gov. The ACT intervention showed promising results on cessation processes and outcomes at short term follow-up (i.e., 3 months). Building on these results, a randomized trial with a larger sample size (N = 2554) and longer term follow-up (i.e., 12 months) is now needed to definitively determine whether the ACT website provides more robust quit rates than currently available smoking cessation websites. Accordingly, we propose to conduct a randomized controlled trial comparing the ACT smoking cessation website with Smokefree.gov, in order to: (1) Demonstrate that web-based ACT has significantly higher smoking cessation rates than a national-level current standard website (Smokefree.gov). Primary outcome: 30-day point prevalence abstinence at 12 months post randomization; (2) Demonstrate that the web-based ACT (but not Smokefree.gov) smoking cessation outcomes are mediated by three psychological processes that are central to the theoretical model underlying ACT: acceptance of internal (a) sensations, (b) emotions, and (c) thoughts that cue smoking. This study, well-motivated by promising pilot results, will provide a definitive test of the web-based ACT for smoking cessation program and its underlying mediational processes. Positive results could provide current standard web-based interventions a new intervention paradigm and significantly improve long term cessation rates for smokers in a low cost public health intervention.